1. Field of the Invention
The invention is directed to dispersion compensating fibers for use in telecommunication systems, and more particularly, to optical fibers for compensating the dispersion and dispersion slope of non-zero dispersion shifted fibers.
2. Technical Background
The increased demand for higher bit transmission rates has resulted in a large demand for optical transmission systems that can control dispersion effects. A linear analysis of common optical transmission systems indicates that while transmission systems can tolerate about 1,000 ps/nm residual dispersion at 10 Gbit/second, these systems tolerate only about 62 ps/nm residual dispersion at 40 Gbit/second. Therefore, it is apparent that it is important to accurately control the dispersion within high bit-rate transmission systems, and that this control becomes increasingly important as the transfer rate increases. Further, the need to accurately control dispersion means that dispersion slope of a transmission fiber must also be compensated as transfer rates approach 40 Gbit/second.
Various solutions have been proposed to achieve the low dispersion and dispersion slope values required for compensating non-zero dispersion shifted fibers, including: photonic crystal fibers, higher order dispersion compensation, dispersion compensating gratings and dual fiber dispersion compensating techniques. Each of these solutions has significant drawbacks associated therewith.
Photonic crystal fibers are designed to have a large negative dispersion and a negative dispersion slope that are close to those required for compensating non-zero dispersion shifted fibers. However, photonic crystal fibers have significant drawbacks including a relatively small effective area of about 10 μm2 or less that leads to unacceptably high splice losses and hence require the use of a transition fiber to reduce splice losses. In addition, due to the very nature of photonic crystal fibers, i.e. glass/air interfaces in the core of the fiber, the related attenuation is unacceptable in the transmission window of interest due to the residual absorption of the 1380 nm water peak. Further, photonic crystal fibers are significantly difficult and expensive to manufacture on a large scale.
Higher order dispersion compensation relies on the dispersion properties of higher order modes in multi-mode fiber. It has been demonstrated that higher order modes, e.g. LP02 and LP11, have higher negative dispersions and dispersion slopes than fundamental modes. Higher order dispersion compensation typically relies on the conversion of a transmitted fundamental mode to one of the higher order modes via a mode converter. Subsequently, this higher order mode is propagated in a fiber that supports that mode in addition to the fundamental mode. After a finite distance, the higher order mode is coupled back to the fundamental mode via a second mode converting device. Problems associated with higher ordered dispersion compensation solutions include inefficient mode converters and the difficulty of producing intermediate fibers that allow higher order mode transmission while resisting coupling to the fundamental mode.
Dispersion compensating gratings are utilized to achieve a required differential group delay via chirped gratings. Techniques utilizing dispersion compensating gratings have been shown to be useful only for narrow bands, as these techniques typically suffer from dispersion and dispersion slope ripple when the required grating length becomes large.
Dual fiber dispersion compensating solutions for non-zero dispersion shifted fibers are similar to the dispersion compensating gratings techniques described above in that the dispersion compensation and the slope compensation are de-coupled and solved for separately. Typically, dual fiber dispersion compensating techniques include the use of a dispersion compensating fiber followed by a dispersion slope compensating fiber. Such solutions require the use of a dispersion slope compensating fiber that compensates for a relatively small dispersion slope. Extensive profile modeling of optical fibers has resulted in well-established correlations between dispersion slope, effective area and bend sensitivity. By increasing the role played by waveguide dispersion in a given fiber, it is possible to decrease the slope and even create a negative slope in some cases. However, as the effective area is decreased, the bend sensitivity of the fiber is increased. Effective area of the fiber can be increased at the expense of further degradation of the bend sensitivity. Decreasing the dispersion slope, or making the dispersion slope negative, results in working very close to the cut-off wavelength of the fundamental mode, which in turn makes the fiber more bend sensitive and results in greater signal loss at long wavelengths, i.e., wavelengths greater than 1560 nm. As a result of these relationships, it is extremely difficult to manufacture a viable slope compensating fiber for the two fiber dispersion and dispersion slope compensating solutions.
Heretofore, the most viable broad band commercial technology available to reduce or eliminate dispersion has been dispersion compensating fiber modules. As dense wavelength division multiplexing deployments increase to 16, 32, 40 and more channels, broadband dispersion compensating products are desired. Telecommunication systems presently include single-mode optical fibers which are designed to enable transmission of signals at wavelengths around 1550 nm in order to utilize the effective and reliable erbium-doped fiber amplifiers currently available. One such fiber is LEAF® optical fiber, manufactured by and available from Corning, Inc. of Corning, N.Y. LEAF® fiber is a positive non-zero dispersion shifted fiber that has become the optical fiber of choice for many new systems due to its inherently low dispersion and economic advantage over conventional single mode fibers.
With continuing interest in higher bit-rate information transfer, i.e. greater than 40 Gbit/second, ultra-long reach systems, i.e., systems greater than 100 km in length, and optical networking, it has become imperative to use dispersion compensating fibers in networks that carry data on non-zero dispersion shifted fibers. The combination of the early versions of dispersion compensating fibers with non-zero dispersion shifted fibers effectively compensated dispersion at only one wavelength. However, higher bit-rates, longer reaches and wider bandwidths require dispersion slope to be compensated more exactly. Consequently, it is desirable for the dispersion compensating fiber to have dispersion characteristics such that its dispersion and dispersion slope are matched to that of the transmission fiber it is required to compensate. The dispersion and dispersion slope matching characteristic described above is also indicative of multiple dispersion compensating fiber systems.
It would therefore be desirable to develop an alternative dispersion compensating apparatus having the ability to compensate for dispersion of non-zero dispersion shifted fibers and other positive dispersion optical fibers over a wide wavelength band around 1550 nm.